Stents are generally cylindrical shaped devices that are radially expandable to hold open a segment of a blood vessel or other anatomical lumen after implantation into the body lumen. Stents have been developed with coatings to deliver drugs or other therapeutic agents.
Stents are used in conjunction with balloon catheters in a variety of medical therapeutic applications including intravascular angioplasty. For example, a balloon catheter device is inflated during PTCA (percutaneous transluminal coronary angioplasty) to dilate a stenotic blood vessel. The stenosis may be the result of a lesion such as a plaque or thrombus. After inflation, the pressurized balloon exerts a compressive force on the lesion thereby increasing the inner diameter of the affected vessel. The increased interior vessel diameter facilitates improved blood flow. Soon after the procedure, however, a significant proportion of treated vessels re-narrow.
To prevent restenosis, short flexible cylinders, or stents, constructed of metal or various polymers are implanted within the vessel to maintain lumen size. The stents acts as a scaffold to support the lumen in an open position. Various configurations of stents include a cylindrical tube defined by a mesh, interconnected stents or like segments. Some exemplary stents are disclosed in U.S. Pat. No. 5,292,331 to Boneau, U.S. Pat. No. 6,090,127 to Globerman, U.S. Pat. No. 5,133,732 to Wiktor, U.S. Pat. No. 4,739,762 to Palmaz and U.S. Pat. No. 5,421,955 to Lau. Balloon-expandable stents are mounted on a collapsed balloon at a diameter smaller than when the stents are deployed. Stents can also be self-expanding, growing to a final diameter when deployed without mechanical assistance from a balloon or like device.
Drug eluting stents currently employ exterior coatings with or without polymers on metal struts to hold a drug for subsequent elution and delivery of the drug to surrounding tissue. Unfortunately, such coatings present a number of problems and limitations. The coatings are fragile and can fracture and fragment during manufacture, delivery, deployment, or use. Fracture during manufacture increases the cost and complexity of manufacture. Fracture during delivery, deployment, or use can reduce the effectiveness of the stent due to lost drug and can pose a risk to the patient if fragments block blood flow. The drug elutes from the coating surface, so the duration of drug elution is limited by the coating thickness, i.e., the mean diffusion length of the drug within the polymer coating. In addition, metal struts can fatigue and cracks can propagate through metal struts during use, fracturing the metal struts and creating fragments which can present a hazard to the patient. Concerns have also been raised over the long-term effects of polymers in contact with the body.
It would be desirable to have a drug eluting folded stent and stent delivery system that would overcome the above disadvantages.